1. Field of the Invention
The starting point of the invention is a pressure medium drive for opening and closing the contacts of a circuit-breaker, containing a pressure medium reservoir, a drive cylinder, a drive piston sliding in the drive cylinder, a mechanism, acted on by the drive piston, to move one of the circuit-breaker contacts which is movable, and a reversing valve for connecting the pressure medium reservoir to the drive cylinder when closing the contacts and for connecting the drive cylinder to an outlet volume when opening the contacts.
2. Discussion of Background
A pressure medium drive of this kind is described in U.S. Pat. No. 2,384,801 A. This pressure medium drive has a piston 17 which is guided in a cylinder 15 and via a piston rod 49 and a linkage 50 acts on a movable contact of a circuit-breaker. To close the contacts of the circuit-breaker, compressed air is fed to the cylinder 15 via an electromagnetically operated reversing valve 21 and a throttle valve 23 from a compressed air reservoir connected to a chamber 186 of the reversing valve 21. The compressed air passed into the cylinder 15 displaces the piston 17 and thus closes the contacts and loads a restoring spring 52 which opens the contacts in the switching-off operation. The reversing valve 21 contains an inlet valve 185, via which the compressed air is passed into the cylinder 15 in the switching-on operation, and also an outlet valve 193, via which air is vented to the atmosphere from the cylinder 15 when opening the contacts with the inlet valve 185 closed. The flow cross sections of the inlet and outlet valves are roughly equal in size and adapted to the period of time needed for closing the contacts and loading the restoring spring 52. In the event of short-circuit switch-off, the time allowed for the opening of the contacts is however substantially shorter than for closing. The pressure medium drive according to the prior art therefore has additionally another electromagnetically operated valve 25 which has a substantially larger flow cross section than the inlet or outlet valve. The time required for opening the contacts is thus substantially shortened. However, a valve of this kind is expensive because of its large flow cross section.